Electron beam discharge device



Nov. 19, 1957 a. H. ROBERTSON 2,813,990

ELECTRON BEAM DISCHARGE DEVICE Filed D80. 30, 1953 N 0 m m we 0 MR N 3 Mv/ M 0N mm H M 6t ww NY "W 6 mm.

' ATTORNEY United States Patent ELECTRON BEAM DISCHARGE DEVICE George H.Robertson, New Providence, N. 1., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication December 30, 1953, Serial No. 401,231

8 Claims. (Cl. 313-85) This invention relates to high intensity electronbeam discharge devices and more particularly to hollow electron sourcestherefor of the type disclosed in the applications Serial Nos. 361,527,361,623 and 361,663, filed June 15, 1953, of D. MacNai'r. I a

In electron tubes which depend for operation upon a small concentratedbeam or pencil of electrons, the provision of an electron source whichis capable of supplying copious emission over a long period of time isof prime importance. Commonly used sources of electrons areequipotential cathodes which include a planar surface coated with amixture of the oxides of barium, calcium and strontium. Of course, thelarger the emissivelythe electron source form a concentrated beam,either comparatively small emissive surfaces have been employed despitethe limitation on quantity of electrons available, or elaborate focusingsystems were used without complete success. The need for providing animproved electron source is indicated by an estimate that 80 percent ofthe failures of cathode ray tubes are due to poor emission. One cause ofpoor emission of the cathode is the gradual evaporation of the electronproducing material, for example, free barium from the limited area ofemissive coating. Another cause of failure of the electron source, whichhas been noted particularly in traveling wave tubes, is bombardment ofthe emissive surface by charged particles, particularly positive ions.Instances have been noted where the back bombardment was so great thatthe cathode was actually punctured and the heater wire burned through,ending the life of the device.

With these difficulties in mind, it is a general object of thisinvention to improve electron sources for electron beam vacuum tubes.

More'specific objects of this invention are to enhance the quantity ofelectron emission of cathodes while insuring long life of the electronsource; to minimize the evaporation of electron emissive material fromcathodes; and to eliminate detrimental bombardment of the electronsource.

These objects are attained in accordance with this invention, onespecific illustrative embodiment of which comprises a traveling wavetube including a highly evacuated envelope containing an electronsource, a helical electrode and a collector electrode. Electromagneticwave input and output transmission lines are coupled to the particles inthe region of the emitted beam of electrons.

ICC

A feature of this invention involves the configuration of the electronsource whereby a large reservoir of electron producing material enclosedwithin the metallic body provides a concentrated beam of electrons froma restricted opening in the body.

In accordance with another feature of this invention no portion of theelectron emitting surface is exposed to bombardment by positive ions.

Another feature of this invention relates to the inclusion of anaperture in the cathode in the path of positive ion bombardment.

Still another feature of this invention relates to the presence of apositive ion collecting electrode opposite the aperture in the electronsource.

A more complete understanding of this invention may be had from thefollowing detailed description and by reference to the accompanyingdrawings in which:

Fig. 1 is an axial sectional view of a traveling wave tube embodyingthis invention;

Fig. 2 is a magnified view of the electron source of the traveling wavetube of Fig. 1; and

Fig. 3 is a schematic representation of the electron and positive ionpaths within the device of Fig. 1.

Referring now to Fig. 1, one specific illustrative embodiment, atraveling wave tube, may be seen. It comprises a highly evacuatedmetallic housing 10 including a centrol bore 11 therein, a cathode basemember 12 mounting a vitreous disc 13 including terminal pins 14extending sealed therethrough, and a collector base member 16 includingan exhaust tubulation 17. Within the bore 11 is a helix 19 mounted froma series of insulating rods 20, for example, three, which are disposedin a circular arrangement and extend between an accelerating grid 21 andan electron collector 35. The ends of the helix 19 are electricallyconnected to the center conductors 41 and 42 of coaxial input and outputtransmission lines 43 and 4-4. Mounted from a stepped vitreous ring 25adjacent grid 21 is an electron source or cathode generally designated26. It comprises a metallic body 27, for example, of nickel, encompassedby a heater wire 28, a heater shell 29 and a heat shield 30. The heatshield 30 fits snugly within the stepped ring 25, and the heater shell29 and metallic body 27 are suspended from the shield 30 by a series offingers 31, one of which is shown. The cathode 26 is electricallyconnected to one of the terminal pins 14 by a nickel lead as are theends of the heater wire 28. Aligned with cathode 26 at the opposite endof helix 19 is the electron collector 35 in position to receiveelectrons emitted from cathode 26 after they have pased through thehelix 19. On the opposite side of the cathode 26 from the helix 19 andthe electron collector 35 is a positive ion collector 36 in the form ofa solid block of nickel.

The details of cathode 26 may be seen more clearly in Fig. 2. Themetallic body 27 comprises a pair of recessed cylinders 37 securedtogether as by welding. The recesses form a toroidal cavity 32 withinthe body 27. A sprayed or compressed coating 38 of electron emissivematerial, a mixture of the oxides of barium, calcium and strontium,covers the recessed surfaces of the cylinders 37 forming the cavity 32.As an exit for electrons emitted from the enclosed cavity 32, the body27 includes a central aperture 33, which is coaxial with the toroidalcavity 32, and an annular passage 34 communicating between the cavity 32and the aperture 33. The coating 38 constitutes an electron reservoir ofsubstantial area compared with the ultimate emitting orifice, aperture33 or to the annular passage 34 both of which tend to concentrate theelectron flow.

Electron emission is achieved when the coating is heated by the heaterwire 28 and electrons pass through the sive surface.

passage 34 and are drawn from aperture 33 by an accelerating field, inthis embodiment supplied by accelerating grid 21.

vIn Fig. 3 the relative position of the electrodes of the device of Fig.1 are shown along with a representation of the paths of electrons fromthe cathode 2d and of positive ions originating in the electron stream.The electron stream is indicated by dotted lines emerging from thetoroidal cavity 32, passage 34 and aperture 33 to be drawn to theelectron collector 35. The electron stream is shown as diverging butthis characteristic may be modified by auxiliary electrodes if desiredor by the cathode itself where .the diameter and the length of aperture33 are correlated, as taught in the application Serial No. 361,527mentioned heretofore. Positive ions shown in Fig. 3 as solid linestravel toward the cathode 26 being concentrated largely within thebounds of the electron beam. Owing to the configuration of cathode 26,substantially all of the positive ions within the electron beam passcompletely through the aperture 33 and may advantageously be collectedby the ion collector 36. Any positive ions drawn to the cathode outsideof the confines of the electron beam will strike its uncoated exteriorsurface and can do no damage to the emissive coating. Electrons easilytraverse the path from the cavity 32 through the passage 34 and outofaperture 33. Positive ions on the other hand having a mass many timesgreater than that of the electrons deviate slightly, if at all, from astraight-line path from their point of origin in the electron stream tothe cathode, their motion terminating at either the ion collector or thecathode surface. In operation the ion collector 36 and cathode 26 may beelectrically connected together to operate at the same potential or,advantageously, the ion collector may be biased negative with respect tothe cathode.

The electron source in accordance with this invention is disclosed as apart of a traveling wave tube to indicate the relative positions of thecathode and ion collector. Its'application, of course, is not limited totraveling wave tubes or devices employing an ion collector, but it maybe readily applied to cathode ray tubes or electron beam type tubesgenerally.

As was explained in the related applications identified above, thehollow cathodes as disclosed therein produce high intensity electronbeams at low accelerating electrode potential. In accordance with thisinvention not only is a high current density obtained, but longer lifeof the electron source results. The mechanisms by which copious electronemission for a long period of time is obtained from the electron sourceof this invention are not fully understood but by way of explanation itis suggested that, first, an extremely large emissive area is presentwithin the body 26 supplying electrons through a compara- .tively smallopening readily heated to emissive temperature by the heater wire 28 andthe toroidal-disposition of the emissive surface minimizes the retardingeifect of electron space charge upon cathode emission.

According to conventional emission theory the confined arrangement ofthe electron emitting material in cathodes of this invention would giverise to a high space charge within the cavity 32 severely limiting theelectron current available. Such is not the case. It is believed that inthe circular cross-sectional cavity the point of highest space chargeWould be the geometrical center but owing to the discontinuity in thecavity wall due to the presence of passage 34, the normal space chargedistribution is disturbed resulting in a space charge minimum in theregion of the passage 34. Electrons emerging from the coating 38 withsufficient thermal energy can reach the space charge minimum adjacentpassage 34 and travel therethrough to come under the eiTect of theaccelerating field.

Secondly, loss of portions of the emissive coating, particularly freebarium, due to evaporation or migration is greatly restricted owing tothe confined nature of .the emis- Further'more, emissive coatingmigrating all) to the passage 34 and to aperture 33 would still beavailable for supplying electrons, and the rate of deposition on otherelectrodes is greatly reduced.

Thirdly, damage to the cathode and emissive coating due to positive ionbombardment is virtually eliminated.

It is to be understood that the above described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scopeof the invention.

What is claimed is:

l. A high intensity electron beam source for an electron dischargedevice comprising a metallic body including an electron exit apertureextending therethrough, said body having a toroidal electron reservoirtherein bounded by a substantially unipotential surface and disposedabout said electron exit aperture and communicating therewith, andheater means for raising said reservoir to electron emissiontemperature.

2. A high intensity electron beam source for an electron dischargedevice comprising a metallic body including a central aperture extendingtherethrough, said body including an interior surface defining atoroidal cavity encircling said central aperture and communicatingtherewith through a restricted passage, an electron reservoir coating onsaid surface, and heater means for raising said coating to electronemissive temperature.

3. A high intensity electron beam source for an electrondischarge devicecomprising a metallic body including an interior surface defining atoroidal cavity, a coating of electron emissive material uponsubstantially all of said surface, said body including a centralaperture extending therethrough along the axis of the toroidal cavityand a passage communicating between the emissively coated interiorsurface and the central aperture, and a heater for said body.

4. A high intensity electron beam source for an elec tron dischargedevice comprising a metallic body including an aperture therethrough andan interior surface defining a toroidal cavity encompassing saidaperture and communicating therewith by means of an annular passages, acoating of electron emissive material upon portions of said interiorsurface, and a heater for said body.

5. A high intensity electron beam source for an electron dischargedevice comprising a metallic body including a planar face and anelectron exit aperture extending through said body in the directionsubstantially perpendicular to said planar face, an electron reservoirtoroidally disposed about the electron exit aperture and communicatingwith said aperture through a restricted annular passageway in said body,and a heater for said body.

6. A high intensity electron beam source in accordance with claim 5wherein said electron reservoir comprises a coating of electron emissivematerial upon a toroidal internal surface of the metallic body coaxialwith the electron exit aperture therein.

7. An electron discharge device comprising a highly evacuated envelope,an electron source comprising a metallic body including an electron exitaperture extending therethrough, an electron reservoir within said bodyand disposed about said electron exit aperture and communicatingtherewith, heater means for raising said reservoir to electron emissiontemperature, an electron accelerating electrode spaced from saidelectron source and in alignment with the electron exit aperturetherein, and a positive ion collector electrode spaced from saidelectron source on the opposite side from said electron acceleratingelectrode and in alignment with the electron exit aperture in saidelectron source, said electron reservoir being enclosed within saidmetallic body and shielded thereby from said electron acceleratingelectrode and said positive ion collector electrode.

8. An electron discharge device electrode assembly comprising a metallicbody including an electron exit aperture extending therethrough, anelectron reservoir within said body and disposed about said electronexit aperture and communicating therewith, heater means for raising saidreservoir to electron emission temperature, a positive ion collectorelectrode, and means mounting said metallic body, and means mountingsaid positive ion collector electrode in alignment with the aperture insaid metallic body.

1,959,500 Rogowski May 22, 1934 6 Holst Feb. 12, 1935 Smith May 21, 1940Pierce Feb. 12, 1952 Heil Mar. 25, 1952 Hull Mar. 17, 1953 Field July14, 1953

